1. Field of Invention
This invention relates to coating compositions containing colloidal silica, alkoxysilanes, and acidifying agents, to methods of preparation and use, and to coated articles therefrom.
2. Description of the Prior Art
Attempts have been made to provide stable coating compositions based on aqueous colloidal silica, alkoxysilanes and various acids which cure to adherent crack and temperature resistant, glossy coatings. However, achievement of these goals has been elusive, especially the attainment of stable compositions which are pigmented and yet provide cured coatings which are highly adherent, for example to a metal substrate, are highly temperature resistant, and exhibit a high degree of gloss.
U.S. Pat. No. 3,976,497 to Clark relates to a paint composition containing a pigment carried in a vehicle consisting essentially of a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH).sub.3 in which R is selected from specified groups at least 70 weight percent of the silanol being CH.sub.3 Si(OH).sub.3. The vehicle contains 10 to 50 weight percent solids consisting essentially of 10 to 70 weight percent colloidal silica and 30 to 90 weight percent of the partial condensate. The vehicle further contains sufficient acid to provide a pH in the range of 3.0 to 6.0. The acids disclosed as useful include such inorganic and organic acids as hydrochloric, acetic, formic, propionic, toluenesulfonic and oxalic among others. These reference compositions while providing coatings having improved abrasion resistance suffer from some disadvantages. For example, the stability of these reference compositions is inadequate. Additionally, coatings prepared from these reference compositions tend to lose adhesion from a steel substrate after curing if applied so as to produce dry film thicknesses greater than about 0.5 mils (1.27.times.10.sup.-5 meters).
U.S. Pat. No. 4,159,206 to Armbruster et al is directed to compositions having improved craze resistance and weatherability over compositions of the type disclosed in U.S. Pat. No. 3,976,497. The compositions of U.S. Pat. No. 4,159,206 do not contain pigment. These unpigmented compositions contain colloidal silica and a mixture of dialkyldialkoxysilane and alkyltrialkoxysilane such as dimethyldimethoxysilane and methyltrimethoxysilane prepared by adding a mixture of dialkoxysilane and trialkoxysilane to colloidal silica hydrosols and adjusting the pH with sufficient acid to provide a pH in the range of 3.0 to 6.0. The acids disclosed as useful for adjusting the pH in U.S. Pat. No. 4,159,206 are the same as those disclosed as useful for adjusting the pH in U.S. Pat. No. 3,976,497.
U.S. Pat. No. 3,986,997 to Clark relates to a pigment-free coating composition containing essentially the same components as in U.S. Pat. No. 3,976,497 except without pigment.
U.S. Pat. No. 4,197,230 to Baney et al relates to a modification in the pigment-free compositions shown in U.S. Pat. No. 3,986,997 to Clark by incorporation into the Clark composition of at least 1 weight percent of .phi. Si(OH).sub.3 based on the weight of total RSi(OH).sub.3 present in the composition.
The present invention is directed to compositions based on a dispersion of colloidal silica, water and alkoxysilanes which have improved stability and can provide coatings which are temperature resistant, water resistant, crack resistant, highly adherent, and/or glossy.